INFLUENCES OF NITROGEN NUTRIENT AND SOLAR-RADIATION IN THE CANOPY ON LENGTH OF LOWER INTERNODES OF RICE

Factors for causing variations in lower internodes length of rice were investigated in relation to lengths of synchronously extending leaf blade, LAI and radiation environment in the canopy for a cultivar 'Nipponbare' grown under different split application conditions of N fertilizer. The N application conditions included light basal and heavy top-dressings from the panicle initiation stage onward (L), and heavy basal and heavy top-dressings from the spikelet formation stage onward (V). Lengths of lower internodes showed no or negative correlations with those of upper leaf blades which were synchronously extending with the internodes. Under the same amount of total N application, the lengths of lower internodes at L were shorter than those at V. Since N top-dressing at L was applied at the lower internodes elongation stage, N is not likely to bc the direct cause for the lower internodes elongation. Lengths of lower internodes correlated positively with LAI at 30 days before heading, and negatively with accumulated solar radiation at the stem base during the lower internodes elongation stage. Shading treatment given to the stem base of the plants elongated lower internodes. These results indicate that the primary factor for the variation of lengths of lower internodes in rice is solar radiation conditions in the canopy through the promotions of LAI development.